As an example of cutting inserts, Japanese Unexamined Patent Publication No. 2008-544872 discloses a reversible cut insert having a square shape in a top view. This publication also describes that the cutting insert performs a cutting process of a workpiece by using major edges of major side surfaces, respectively, corresponding to individual sides of the square shape, and auxiliary edges of auxiliary side surfaces formed by cutting out corner parts.
However, the foregoing cutting insert is configured to cut a workpiece surface by using the auxiliary edges of the auxiliary side surfaces having a planar shape, thus causing the problem that it is difficult to ensure a sufficient flank angle between the auxiliary side surfaces and the workpiece surface. Hence, there is a risk that the contact between the auxiliary side surfaces and the workpiece surface will deteriorate surface finish of the workpiece surface, and the auxiliary side surfaces will wear out, failing to exhibit a desired function when the cutting insert is used in a reversible manner.
On the other hand, in order to reduce the contact between the auxiliary side surfaces and the workpiece surface, it is necessary to decrease an axial rake angle of the major edges of the major side surface when the cutting insert is attached to a holder. Thus, there is a risk that the cutting resistance exerted on the major edges will be increased. There is also a concern that the cutting insert will be lifted up from the holder in a predetermined direction by a cutting force exerted on the cutting insert when the workpiece surface is cut by the auxiliary edges of the auxiliary side surfaces.